Rename `*_user_groups()` methods to `*_user_groups_having()`

In preparation of introducing `*_viewing()` methods so we can distinguish user groups that have the profile fields (data) versus user groups that can view the profile fields.
Introduce *_viewing() methods to get/update the viewing user groups
Introduce can_user_view_fieldgroup() and can_user_view_field() methods
Implement viewing user groups' admin metaboxes for both fieldgroups and fields
Fix updating fieldgroup meta, correctly check metabox nonces and apply can_user_view_* methods to access filters
Undo the users-without-groups viewing option
Rename metabox title to 'User Groups'
Fix updating metabox values + better describe metabox input lists
Correct docblock headers for new 1.1.0 methods
Implement singleton pattern
Update readme to reflect recent changes before next release